Not all men cheat
And hell, not all women are that great at not cheating
But, this is besides the point. I will lay down two potential options
1. COMMUNICATE - lay down some ground rules. Shannon is not entirely cool with me sleeping with someone else in our bed, especially if Shannon is either home or will be soon! Whereas I couldn't give a toss either way. Getting it on whilst I'm in the study? Well, either let me join in or make sure you've left me the good headphones if I'm trying to study.
2. DITCH AND WORK ON YOURSELF - Go work on your self-esteem. Having a good self-esteem often reaps wonders in our relationships. It sounds corny but try the following before you go to sleep each night "Ten good things that happened today" and "Ten things I like about myself".
If you cannot do ten, don't worry. Start with one and work your way up.
And whenever you start getting these little doubts throughout the day, start repeating them to yourself.